Early B cell Factor (EBF) is an essential regulator of genes in B lymphocytes. EBF drives B cell development from early progenitors to mature B cells by activating genes encoding the pre-B and mature B cell receptors (pre-BCR and BCR). In mice lacking EBF, B cell development is arrested, immunoglobulins are not produced and EBF target genes are not activated. In the absence of EBF mb-1 (Ig-a) genes remain in hypermethylated, inaccessible chromatin. Recently, we demonstrated that EBF is required for, and indeed, initiates CpG demethylation and remodeling of nucleosomes at the B cell-specific mb-1 promoter. Remodeling of chromatin by EBF facilitates binding of the promoter by the B lineage commitment factor Pax5. Our studies identified a new 'functional'hierarchy of transcription factors that acts in addition to the 'genetic'hierarchy identified previously (EBF regulates expression of pax5). We have proposed that B lineage determination by EBF is largely a function of its essential role as a chromatin remodeling 'pioneer factor'. To understand how EBF drives B lineage determination, we propose to 1) identify gene targets of EBF that are important in B lineage specification, 2) determine how EBF initiates demethylation of the mb-1 promoter and 3) determine the molecular basis of EBF-dependent chromatin remodeling.